1. Field of the Invention
The present invention relates to an exhaust emission control device which controls regeneration of a diesel particulate filter to remove particulate matters contained in exhaust gas of an internal combustion engine and deposited in the filter.
2. Description of Related Art
For the environmental protection, it is necessary to purify exhaust gas outputted from an internal combustion engine of a vehicle. For example, it is necessary to remove particulate matters from exhaust gas of a diesel engine. To remove particulate matters, a diesel particulate filter (hereinafter, called DPF) is disposed in an exhaust pipe through which the exhaust gas outputted from the engine flows. The DPF normally has a filter formed in a honeycomb structure. This honeycomb filter catches and collects a major portion of particulate matters outputted from the engine, so that the exhaust gas is purified.
However, each time a certain quantity of particulate matters are deposited in the DPF, it is necessary to burn the deposited particulate matters for the purpose of regenerating the DPF. As a technique for burning the particulate matters, post injection of fuel is well known. In this post injection, fuel is injected into the engine at a timing retarded from a timing of the normal main injection of fuel.
As the post injection, both multi-post injection and single-post injection are known. In the multi-post injection, a series of fuel injections is performed after the main fuel injection. In the single-post injection, only one fuel injection is performed after the main fuel injection. In case of the multi-post injection, the combustion of fuel is continued in cylinders of the engine to rise the temperature of exhaust gas outputted from the engine. The temperature of the DPF receiving this exhaust gas is risen, so that particulate matters of the DPF are burned. That is, the DPF is purified in response to the temperature rise based on the exhaust gas (hereinafter, called exhaust gas-based temperature rise).
In contrast, in case of the single-post injection, a major portion of fuel injected in the post injection is not burned in the engine, so that unburned hydrocarbons are outputted from the engine and are fed to the DPF. In the DPF, the hydrocarbons are oxidized due to the catalytic reaction caused by catalyst of the DPF, so that the temperature of the DPF is risen by heat generated in the reaction of the hydrocarbons. Therefore, particulate matters of the DPF are burned. That is, the DPF is purified in response to the temperature rise based on hydrocarbons (hereinafter, called hydrocarbon-based temperature rise).
FIG. 1A shows the relationship between the injection valve lift position and the heat release rate in a diesel engine in case of the exhaust gas-based temperature rise, while FIG. 11 shows the relationship between the injection valve lift position and the heat release rate in a diesel engine in case of the hydrocarbon-based temperature rise.
As shown in FIG. 1A and FIG. 1B, main injection is performed at a timing of compression top dead center (TDC). After the main injection, multi-post injection or single-post injection is performed in a period of time between TDC and after top dead center 90 (ATDC90) Heat is generated in an engine in response to the multi-post injection, so that the temperature of exhaust gas is heightened. In contrast, no heat is substantially generated in response to the single-post injection, so that unburned hydrocarbons are outputted from the engine.
When particulate matters are deposited in the DPF, the particulate matters are often deposited in layers on the catalyst held on the front end surface of the DPF. In this case, it is difficult to burn the particulate matters deposited on the front end surface of the DPF by oxidizing unburned hydrocarbons. Therefore, to burn the particulate matters deposited on the front end surface of the DPF, it is required to heighten the temperature of the exhaust gas passing though the DPF.
In the exhaust system holding the catalyst on the upstream side of the DPF, the temperature of the exhaust gas is sometimes risen in response to the oxidation of hydrocarbons based on the catalytic reaction. Therefore, to burn the particulate matters deposited on the front end surface of the DPF, it is not necessary to heighten the temperature of the exhaust gas outputted from the engine. In contrast, in the single DPF system holding no catalyst on the upstream side of the DPF, to burn the particulate matters deposited on the front end surface of the DPF, it is indispensable to heighten the temperature of the exhaust gas outputted from the engine.
In the hydrocarbon-based temperature rise, unburned hydrocarbons not burned in cylinders of the engine are fed to the DPF and are oxidized based on the catalytic reaction, so that the temperature of the DPF is risen. Therefore, the temperature of the exhaust gas outputted from the engine is generally low. In contrast, in the exhaust gas-based temperature rise, fuel injected in the multi-post injection is continuously burned in the engine, so that the temperature of the exhaust gas is heightened. Therefore, the temperature of the exhaust gas outputted from the engine is high. Therefore, for the regeneration of the DPF in the single DPF system, the exhaust gas-based temperature rise is often used.
However, in the exhaust gas-based temperature rise, the heat of the exhaust gas outputted from the engine and flowing through the exhaust pipe is easily dissipated to the outside through the exhaust pipe before the exhaust gas is fed to the DPF. Therefore, to give the dissipated heat and the regeneration heat to the exhaust gas outputted from the engine, it is required to inject a large quantity of fuel in the post injection. In this case, because the temperature of the exhaust gas outputted from the engine is sufficiently heightened to reliably rise the temperature of the DPF, fuel is excessively consumed. Therefore, fuel economy in the vehicle deteriorates.
In contrast, in the combustion of the particulate matters deposited on the front end surface of the DPF, the hydrocarbon-based temperature rise is inferior to the exhaust gas-based temperature rise. However, to rise the temperature of the whole DPF, the hydrocarbon-based temperature rise is superior to the exhaust gas-based temperature rise. That is, in case of the hydrocarbon-based temperature rise, the temperature of the DPF is rapidly risen so as to rapidly regenerate the DPF, so that the deterioration of fuel economy can be suppressed. In the prior art, because only the exhaust gas-based temperature rise is used to regenerate the DPF, the merits of the hydrocarbon-based temperature rise are not obtained.
Assuming that an exhaust emission control device appropriately controls the regeneration of the DPF while considering the merits and demerits in both the exhaust gas-based temperature rise and the hydrocarbon-based temperature rise, the temperature of the DPF is rapidly risen, and fuel consumption in the DPF regeneration is suppressed.
For example, Published Japanese Patent First Publication No. 2007-23961 discloses a fuel injection control device. In this device, to improve the durability of the engine and to lengthen the maintenance interval, the dilution of oil caused by the usage of both the exhaust gas-based temperature rise and the hydrocarbon-based temperature rise is suppressed. More specifically, in response to engine conditions, the post injection for the exhaust gas-based temperature rise is performed for some of cylinders of the engine, and the post injection for the hydrocarbon-based temperature rise is performed for the other cylinders of the engine. That is, the injection mode is set for each cylinder to operate the cylinders according to different injection modes.
However, the prior art including the Publication No. 2007-23961 does not teach or even suggest a technique for alternately selecting the exhaust gas-based temperature rise and the hydrocarbon-based temperature rise to rapidly regenerate the DPF in the single DPF system holding no catalyst on the upstream side of the DPF.